Echo suppressor



Aug. 12, 1930.

D. MrrcHz-:LLKET AL ECHO SUPPRESSOR Filed Feb. 20. 1929 l l l I l l I l I l I Echo with JJezayD I l l Ptentled Aug. 12, '1930 UNITED sTAT-as ;PAT-EN? oFFic-E DOREN MITCHELL, OF 'NEW YORK, N.. Y., ,ANDJOSEPH HERMAN, OF WESTFIELD, NEW

masrm-AssrGNon-TOAMERICAN 'rELErHoNE AND TELEGRAPH COMPANY, A cona POBATION OF NEW YORKfli-W norm sprmmssoa- V V Application led February 20, 1929. Serial No. 341,544,

This inven'tionrelates'to two-way transw in the art, reference may be had'to an article l' 47by A. B. Glark and R. C. Mathes, Journal .'of the American Institute of Electrical Engineers, June, 1925, pages 618-626.

It is often necessary, especially in long twoi way circuit's, to provide"`a'hang-over or slow release ;period' of considrable length for ag transmission circuit blockin'g'relay of the echo suppressorfin order 'to 'provide full echo sup'preslsion and toW prevent false operation rof. the 'opposite blocki-ngi-relay. 'This hangoverperiod may render it diflicult for the lis- 'tener' to interrupt `the speaker. The time during which the return path is blocked may be greatly reduced and breaking made easier if a delay is introduced inthe ec io suppressor circuit such that a transmission circuit is Vblocked only while an echo is actually present in the system. 'If a delay is introduced in the echo suppressor circuitahead of the amplifier-detector a voice frequency delay circuit is reglllired, and such a device is quite expensive. e object of' the present lnvention is to introduce the desired delay in the echo suppressor circuit without the requirement of the expensive voice frequency delay circuit.

'In attainin'g this object the applicants take the voice energy from one side of theusual four-wirecircuit, detect this energy, delay the transmission through the echo suppre'ssoi' cir- 40 cuit of the directcurrent component and then apply the delayed current to block the opposite side of the four-Wire circuit. p The invention will now be clearly understood When the following detailed description of one suitable system for accomplishing the desired result isread with reference to the accompanying drawing, which shows diagrammatically, and in part schematically, the echo suppressor circuits applied.

With reference to the details of the drawing, there is shown a sectiony of .a four-Wire circuit havingatwo-wire line L adapted for transmission from west to east, for instance, and a secondtwo-Wire line Lf ada ted for transmission from east to west. ne-Way amplifiers A and A' are included in the lines L and L', respectively; It is to be understood that the echo suppressor is so located with 'respect to the ends of the four-Wire circuit that transmission over line may produce van ech o which returns to the echo suppressor point mline L' With a delay D, while transmission over line L' `may produce an echo returning to the echosup ressor point in line L with a different delay I'. i

An amplifier-detector AD is connected across the line L; als; shown inthe drawing. This amplifier-detector functions ultimately to operate a relay R2 which places a short- `circuit on the line L'. It is the applicants,

be employed, but such a device also is cumber--` some.. No mechanical-a delay device has yet been developed which would be reliable for the present purpose. Accordingly, the appli- .cants employ the following novel circuit arrangement whereby a low frequency delay Circuit may be used. a

The amplifier-detector A'D operates, in response to Waves in line L,a rela R1 and also a relay R3, to be discussed heremafter. The relayl R1 upon ,o eration closes a circuit through abattery a resistance 7', a low frequency delay circuit DC, and the winding vof' relay R2. ,The delay circuit DC is madeup of condensers and standard coils of very high-inductance, and can be constructed quitechea ly. This delay circuit has a very low cut-o frequency, 10 or 20 cycles, per- 'haps The delay circuit is adjusted to give the required delay in the operation of relay R2 ;that is, the low frequency component resultin from the detection in the device AD is hel up over a period of time such that the operation of relay R2 will occur just at the instant that the echo of delay D reaches the echo suppressor oint in line L'. It will be noted that relay 1 is slow releasing. In the absence of this slow release feature of relay R1-short impulses of speech, which might cause serious echoes, would not hold re ay R1 long enough to send a current through the delay circuit DC and the echo suppressor would fail to operate relay R2. If, however, a proper hangover is provided for the relay R1, even a veryshort s cech impulse Will cause this relay to transmit through the delay circuit DC an impulse of suflicient length to operate the shorting relay R2. The slow release period should be such that the impulses transmitted through the delay circuit will have a duration of approximately -l at least,

where fa is the cut-ofl frequency of the delay circuit DC.

On the other' side of the four-Wire circuit the amElifier-detector AD' responds to Waves in'line and causes the operation of relay R'1 and a second relay R'a. Relay R'1 upon oper- -ation closes a circuit through'a battery B', a

resistance r', the delay circuit DC', and the winding of relayv R'z, which is the shorting relay for line L. The delay circuit DC' corresponds to the delay circuit DC described above and introduces a delay of the low frequency component corresponding to the delay of the echo returningfromthe West end of the lfour-Wire circuit over line L to the echo suppressor point. tIn order that even a very short impulse may cause the proper operation of relay R'2, the relay R'1 has a delayed release such as that described above in connection with relay R1.

Ifpthe talker at the west end of the four- 'wire circuit begins to speak over line L, and immediately thereafter the talker at the east end begins to speak, the original voice Waves in line may operate amplifier-detector AD' and tend to operate relay R 1 which would cause false operation of relay zR'z. In order to prevent such false operation the rela Ra, which is controlled by the amplifieretector AD, is given a slow 'release period cqual to D, the delay of the echo returning from the east end of the circuit over line L The relay R, thus serves to disable the' suppressor operated from L' and thus prevent false operation of relay R'2 in response to delayed energy in the echo suppressorby breaking the' circuit throu h relays R'1 and R'a. Likewise,

rela B',, aving a slow release period equal tol 'is associated with amplifier-detector AD' and serves to prevent false operation of shorting relay R2 by delayed energy throu h the breaking of the circuit throug relays 1 and R,.

The above .expedients are resorted to in v order thatthe talker whose Waves first reach the suppressor ma take complete control of the device and su er no interruption due to the later talker. This does not, of course, prevent the other talker from speaking between the pauses of the first talker.

Proper attention must be given, of course, to the termination of the delay circuits DC and DC', with a view to the matching of impedances. The resistances r'and r' shown in series with batteries B and B', respectively, or the resistance of the relays R2 and R'z may be adjusted to efl'ect'the proper impedance matching.

While the invention has been disclosed in one embodiment which is deemed desirable, it is to be understood that this specific disclosure is for the purpose of illustration and does not limit the scope of the invention, which is to' be determined from the appended claims.

What is claimed is:

1. In an echosuppressor designed to operate in response to voice Waves in one side of a two-way circuit to block the opposite side of the circuit, the method of obtaining delayed action of the suppressor whichconsistsiin taking the voice energy from one side of the circuit, detecting said energy, delay- 'ing the transmission through the suppressor of 'the direct current component, and then applying the delayed current to block the op- 'posite side of the circuit.

,2.l In a two-way circuit having a first path adaptedfor transmission in one irection and a second ath adapted for transmission in the opposite direction, an amlifier-detector associated with the first patmeans con- -trolled thereby for blocking the second path, and a low frequency delay circuit interposed between the output of said amplifier-detector and said blockln means for delaying the operation of said locking means in response to Waves in the first path.

. 3. In a four-Wire circuit having one path adapted for transmis'sion in one 'direction and a second path adapted for transmission in the opposite direction, an amplifier-detector connected across each of said pa-ths, switching means associated with each path for placing a short-circuit thereon, said switching means being fcon'trolled by the amplifier-detector connected across the opposite path, a low frequencydelay circuit interposed between the outputA of' each amplifier-detecto'r and' the short-circuiting switching means Controlled thereby, and means responsive to the operation of each a1nplifier-detector-for preventing the short-circuiting of the path associated therewith.

` In testimony whereof, we have signed' our names to this specification this 18th day of February, 1929.

DOREN MITCHELL. JOSEPH HERMAN. 

